Catching the 2021 γ-ray flare in the blazar TXS 2013+370
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| Format: | Preprint |
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2025
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| author | Michailidis, Giorgos Traianou, Efthalia Marchili, Nicola Paraschos, Giorgos Filippos Krichbaum, Thomas P. Bach, Uwe de la Parra, P. A. Vergara Kim, Dong-Jin Patiño-Álvarez, Victor Manuel Kadler, Matthias Gurwell, Mark |
| author_facet | Michailidis, Giorgos Traianou, Efthalia Marchili, Nicola Paraschos, Giorgos Filippos Krichbaum, Thomas P. Bach, Uwe de la Parra, P. A. Vergara Kim, Dong-Jin Patiño-Álvarez, Victor Manuel Kadler, Matthias Gurwell, Mark |
| contents | The $γ$-ray-loud blazar TXS 2013+370, a powerful multiwavelength emitter at $z = 0.859$, underwent an exceptional GeV outburst in late 2020-early 2021. In this work, we present full-polarization VLBI imaging at 22, 43, and 86 GHz together with contemporaneous single-dish monitoring (radio and $γ$-rays) to localize the high-energy dissipation site and probe the inner-jet structure and magnetic field topology. The images revealed a compact near-core knot at $r \simeq 40$-$60~μ\mathrm{as}$ contemporaneously with the GeV flare and a flat, core-dominated spectrum ($α\gtrsim -0.5$). The core has strong linear polarization and exhibits a $\sim 50^{\circ}$ EVPA rotation at 86 GHz; pixel-based and integrated fits yield a high, uniform rotation measure, ${\rm RM} = (7.8 \pm 0.2) \times 10^{4}~{\rm rad~m^{-2}}$, consistent with an external Faraday screen. Cross-correlation of Fermi-LAT and 15 GHz data shows a significant peak with the $γ$ rays leading by $Δt = (102 \pm 12)\,\mathrm{d}$; adopting $β_{\rm app} = 4.2 \pm 0.5$ and $θ= 4.1^{\circ} \pm 0.2^{\circ}$ implies a de-projected separation $Δr_{γ-15} = (2.71 \pm 0.47)\,\mathrm{pc}$ and locates the GeV emission between the jet apex and $\sim 0.42$ pc ($1σ$ range) downstream. Our results do not uniquely pinpoint the emission site; rather, they support two valid scenarios. The $γ$-ray production may occur within the BLR ($\sim 0.07$pc), where external-Compton scattering of optical/UV photons produces the $γ$ rays, or beyond the BLR, reaching $\sim 0.42$ pc ($1σ$) within the inner parsecs, where external-Compton scattering of dusty-torus infrared photons dominates. Both scenarios are compatible within the allowed range of emission distances, while opacity-driven core shifts modulate the observed radio-$γ$ delay without requiring large relocations of the dissipation zone. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2511_15601 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | Catching the 2021 γ-ray flare in the blazar TXS 2013+370 Michailidis, Giorgos Traianou, Efthalia Marchili, Nicola Paraschos, Giorgos Filippos Krichbaum, Thomas P. Bach, Uwe de la Parra, P. A. Vergara Kim, Dong-Jin Patiño-Álvarez, Victor Manuel Kadler, Matthias Gurwell, Mark High Energy Astrophysical Phenomena Astrophysics of Galaxies The $γ$-ray-loud blazar TXS 2013+370, a powerful multiwavelength emitter at $z = 0.859$, underwent an exceptional GeV outburst in late 2020-early 2021. In this work, we present full-polarization VLBI imaging at 22, 43, and 86 GHz together with contemporaneous single-dish monitoring (radio and $γ$-rays) to localize the high-energy dissipation site and probe the inner-jet structure and magnetic field topology. The images revealed a compact near-core knot at $r \simeq 40$-$60~μ\mathrm{as}$ contemporaneously with the GeV flare and a flat, core-dominated spectrum ($α\gtrsim -0.5$). The core has strong linear polarization and exhibits a $\sim 50^{\circ}$ EVPA rotation at 86 GHz; pixel-based and integrated fits yield a high, uniform rotation measure, ${\rm RM} = (7.8 \pm 0.2) \times 10^{4}~{\rm rad~m^{-2}}$, consistent with an external Faraday screen. Cross-correlation of Fermi-LAT and 15 GHz data shows a significant peak with the $γ$ rays leading by $Δt = (102 \pm 12)\,\mathrm{d}$; adopting $β_{\rm app} = 4.2 \pm 0.5$ and $θ= 4.1^{\circ} \pm 0.2^{\circ}$ implies a de-projected separation $Δr_{γ-15} = (2.71 \pm 0.47)\,\mathrm{pc}$ and locates the GeV emission between the jet apex and $\sim 0.42$ pc ($1σ$ range) downstream. Our results do not uniquely pinpoint the emission site; rather, they support two valid scenarios. The $γ$-ray production may occur within the BLR ($\sim 0.07$pc), where external-Compton scattering of optical/UV photons produces the $γ$ rays, or beyond the BLR, reaching $\sim 0.42$ pc ($1σ$) within the inner parsecs, where external-Compton scattering of dusty-torus infrared photons dominates. Both scenarios are compatible within the allowed range of emission distances, while opacity-driven core shifts modulate the observed radio-$γ$ delay without requiring large relocations of the dissipation zone. |
| title | Catching the 2021 γ-ray flare in the blazar TXS 2013+370 |
| topic | High Energy Astrophysical Phenomena Astrophysics of Galaxies |
| url | https://arxiv.org/abs/2511.15601 |